Hello, my name is***** and I have over 2o years of experience as a veterinarian.
Urine that is noticeably darker yellow in color compared to usual is more concentrated urine, less water in the urine.
There are a number of things that can cause this to happen.
Dehydration is probably the most common, so any disease process that interferes with proper fluid intake and water being absorbed is a possibility.
But increased amounts of bilirubin in the urine (which can be due to liver disease or increased numbers of red blood cells being broken/damaged and their pigments being processed by the liver), an infection, or even dyes in the food can also influence urine color.
If your fellow Diego is feeling poorly, especially if the whites of his eyes look yellow and he is lethargic, running a fever, eating poorly or is vomiting and having bright orange or yellow stools then it is time to see his veterinarian.
I'm sorry I cannot be more specific, but this isn't a characteristic change that points to one disease process, but rather a symptom that encompasses lots of reasons that all lead to dehydration and increased urine concentrations.